<?php
function highlight($sString, $aWords) {
    if (!is_array ($aWords) || empty ($aWords) || !is_string ($sString)) {
        return false;
    }
    $sWords = implode ('|', $aWords);
    return preg_replace ('@\b('.$sWords.')\b@si', '<strong style="background-color:yellow">$1</strong>', $sString);
}